Low loss optical delay line using a microfiber coil

Abstract

An optical delay line is formed from a coil of optical fiber (in many cases microfiber), where the radius of the optical fiber is greater than the wavelength λ of the propagating signal and the radius R of the coil is selected, in consideration with the optical fiber radius, to limit propagation loss by minimizing coupling between adjacent turns of the coil. The difference in dimension between the fiber diameter and wavelength prevents the mode propagating along one turn from coupling into an adjacent turn. It has been discovered that the modal intensity at the interface between the central rod and the coil will be minimized when the radius of the fiber satisfies the following condition: r >> ( R β 2 ) 1 / 3 , where β=(2πn)/λ, and n is the refractive index of the fiber.

1. A low loss optical fiber delay line comprising:
 a section of optical fiber having a diameter 2r greater than the wavelength λ of an optical signal propagating therethrough, the section of optical fiber wound into a coil of length L associated with a predetermined time delay t, the coil exhibiting a radius of curvature selected to satisfy the relation: 
 
       
         
           
             
               
                 r 
                 >> 
                 
                   
                     ( 
                     
                       R 
                       
                         β 
                         2 
                       
                     
                     ) 
                   
                   
                     1 
                     / 
                     3 
                   
                 
               
               , 
             
           
         
       
       where β=(2πn)/λ, and n is the refractive index of the section of optical fiber, the relation between r and R confining a propagating optical mode into an extreme peripheral region of the optical fiber to minimize interaction between adjacent turns of the coil and limit propagation loss. 
     
     
       2. A low loss optical fiber delay line as defined in  claim 1  wherein the section of optical fiber comprises a section of optical microfiber. 
     
     
       3. A low loss optical fiber delay line as defined in  claim 1  wherein the length L of the created optical fiber coil is selected to provide the predetermined time delay t that satisfies the relation:
     t=πnLR/cr,    
 
       where c is the speed of light in a vacuum. 
     
     
       4. A low loss optical fiber delay line as defined in  claim 1  wherein the delay line further comprises
 a central core rod of radius R, with the section of optical fiber wound therearound. 
 
     
     
       5. A low loss optical fiber delay line as defined in  claim 4  wherein the section of optical fiber and the central core rod comprise the same material. 
     
     
       6. A low loss optical fiber delay line as defined in  claim 1 , wherein n=1.5, λ=1.5 μm and r≈0.7 R 1/3 . 
     
     
       7. A low loss optical fiber delay line as defined in  claim 1  wherein the optical fiber comprises a non-uniform radius. 
     
     
       8. A low loss optical fiber delay line as defined in  claim 4  wherein the central core rod comprises a non-uniform radius. 
     
     
       9. A tunable optical fiber delay line comprising
 a plurality of sections of optical fiber having a diameter greater than the wavelength λ of an optical signal propagating therethrough, each section of optical fiber wound into a coil of length L associated with a predetermined time delay t, the coil exhibiting a radius of curvature R selected to satisfy the relation: 
 
       
         
           
             
               
                 r 
                 >> 
                 
                   
                     ( 
                     
                       R 
                       
                         β 
                         2 
                       
                     
                     ) 
                   
                   
                     1 
                     / 
                     3 
                   
                 
               
               , 
             
           
         
       
       where β=(2πn)/λ and n is the refractive index of the section of optical fiber, the relation between r and R confining a propagating optical mode into a region separated from a center area of the optical fiber. 
     
     
       10. A tunable optical fiber delay line as defined in  claim 9  wherein at least one section of optical fiber comprises a section of optical microfiber. 
     
     
       11. A method of fabricating an optical fiber delay line exhibiting a predetermined time delay t, the method comprising the steps of:
 a) providing an optical fiber having a radius r greater that a wavelength λ of an optical signal propagating therethrough; 
 b) selecting a core rod of a radius R that satisfies the relation: 
 
       
         
           
             
               
                 r 
                 >> 
                 
                   
                     ( 
                     
                       R 
                       
                         β 
                         2 
                       
                     
                     ) 
                   
                   
                     1 
                     / 
                     3 
                   
                 
               
               ; 
             
           
         
         c) winding the optical microfiber around the core rod to create a plurality of N turns sufficient to produce the predetermined time delay t. 
       
     
     
       12. The method as defined in  claim 11 , wherein in performing step a), an optical microfiber is provided. 
     
     
       13. The method as defined in  claim 11 , wherein in performing step a), the provided optical fiber comprises a non-uniform radius. 
     
     
       14. The method as defined in  claim 11 , wherein in performing step a), the provided optical fiber comprises a uniform radius. 
     
     
       15. The method as defined in  claim 11 , wherein in performing step b), the selected core rod comprises a non-uniform radius. 
     
     
       16. The method as defined in  claim 11 , wherein in performing step b), the selected core rod comprises a uniform radius.